The invention has application to the distillation of aqueous solutions through microporous hydrophobic membranes whose pore sizes usually range from 0.001 to 10 microns in diameter. The present state of the art is that no manufacturer offers a recommended multiple effect water still which depends on hydrophobic porous barriers and transmembrane evaporation beyond laboratory experimental size.
Practical problems which arise in the design of stills include the high energy use of single effect stills, fouling-induced hydrophilic break-throughs, difficulty in applying feed to the hydrophobic pore entrances, solute polarisation and crystallization in the pores as well as the low rates of diffusion through the long pores of the barriers. These problems have impeded commercial development of membrane distillation.
Some progress has been made in overcoming many of the above problems by coating hydrophobic porous hollow fibre barriers with woven or cast porous coatings which are usually very thin and hydrophilic. However, poor energy efficiency due to difficulty in achieving multistage operation and the low rates of vapour transport through the fibres have remained as prime limitations to commercial acceptance.
The multistage countercurrent flow of hot feed which directly distills into the cold condensate stream is most easily achieved by the use of porous, hydrophobic hollow fibres with very thin walls. There is however, still a difficulty in providing a heat exchanger to recover the heat content of the now hot condensate to reheat the now colder evaporation residue stream. This remains a difficulty and continues to necessitate some form of heat exchanger of large area. There is an economic balance between the cost of the heat energy and the costs added by the heat exchanger.
In some circumstances, the heat is available as waste, but the low rate of vapour transfer through the necessarily fine pores limits the utilization of the otherwise favourable application. The slowness of the vapour transfer is explicable and expected.
Commonly, the tortuous pore path of 200 to 600 microns consists of 0.2 micron diameter pores -- a 1000 to 3000 fold ratio of length to diameter. Some non-condensable or permanent gas is always in the pores and the rates of water vapour diffusion through these gases are low.
Hitherto, the best that could be done was to feed a degassed hot aqueous feed. The diffusing water carried the air entrapped in the pores very slowly out into the cold condensate stream where the air dissolved.
European Patent EP No. 94543 (1983) discloses the methods of heating near to boiling to gain enough vapour pressure to increase diffusion rates. Thermolabile liquids such as milk and beer cannot be treated at high temperatures and cannot be boiled at lower pressures without distilling the volatiles out of the system or reducing the partial pressure of the vapour, which defeats the original purpose of heating to boiling.